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A University of Wyoming faculty member was part of a research team that used global positioning system tracking to understand how interactions between impala (an antelope) and their predators shape patterns of Acacia tree distribution patterns across African savannas.
Jacob Goheen, a UW assistant professor in the Department of Zoology and Physiology, contributed to and advised the Ph.D. student who led a study that demonstrates trees can persist in landscapes characterized by intense herbivory, either by defending themselves with thick thorns or by thriving in risky areas where carnivores, such as leopards and African wild dogs, hunt.
“Given that impala are occurring in this landscape with their predators, the first question we ask is, ‘Do predators scare impala enough that impala won’t go into areas where the predators target their hunting efforts?’ The answer is ‘yes,’” Goheen says. “Impala are remotely controlled by predators, and impala prefer to eat trees with short thorns over trees with long thorns. So, predators are impacting the thorniness of the landscape.”
Goheen is a co-writer of a paper, titled “Large Carnivores Make Savanna Tree Communities Less Thorny,” which was published in today’s (Thursday) issue of Science, an international weekly science journal. Adam Ford, a graduate student at the University of British Columbia, was the paper’s lead writer. Ford was a graduate student there under Goheen, until Goheen came to UW three years ago.
The observation that most ecosystems support abundant plant life -- despite the existence of herbivores that eat plants -- has motivated a great deal of research and debate in ecology. As a result, Goheen and other researchers wanted to understand how plant defense and predation interact across landscapes to shape the distribution of plants in their study area.
Observing the circle of life
The research took place on the Laikipia Plateau, an area of 10,000 square kilometers in central Kenya. Goheen and Ford spent 18 months -- between 2011-13 -- collecting data from a 150-square-kilometer area called the Mpala Conservancy.
Goheen learned that leopards and African wild dogs hunted impala in areas with dense tree cover, where Acacia brevispica, a tree with very short thorns (approximately one-quarter inch in length) was prevalent. The risk of being killed and eaten by leopards and wild dogs keeps impalas out of such areas, and gives the poorly defended trees a buffer against hungry impalas.
On the flip side, impala spend lots of time in open, relatively treeless areas because these areas provide safety. Consequently, there are no Acacia brevispica in these areas because the impala eat them. However the few trees that occur in these safe areas are of a different species (Acacia etbaica) that defends itself with long thorns, approximately 3 inches in length.
“If you think about the life of an impala or any other herbivore, they’re caught between a bit of a rock and a hard place,” Goheen says. “There are a lot of carnivores -- leopards and wild dogs -- that want to eat you. But the things you eat (leaves from Acacia trees) are defending themselves with thorns.”
Researchers tested three hypotheses to explain the structure of this food web.
-- Predation risk drives habitat selection by impala. It was found that impala avoid risky habitats where predators are likely to be present. The group used resource selection functions, GPS collars, telemetry and high-resolution satellite imagery to quantify the selection of woody cover. This represents forage for impala, but could also serve to conceal predators.
-- Impala prefer to eat less-thorny tree species, when given the choice, thereby suppressing their numbers. So, in areas that are safe, impala eat all of the short-thorned trees, leaving only the long-thorned trees.
-- Predation risk increases the numbers of less-thorny Acacia trees, and has no effect on the number of thorny trees. In other words, tree communities became less thorny as predation risk from large carnivores increased.
“What I was surprised to see was that that effect occurs across a large, expansive area. It occurs across the entire study site,” Goheen says. “I didn’t expect the diet of impalas and the fear predators put into impala to translate to which trees occur where across this big of an area.”
Correlations to Yellowstone ecology
Goheen says his interest in such a study stems from similar questions and studies in North America, particularly relative to the effects of wolf reintroduction on elk population declines in Yellowstone National Park.
“These studies are pretty contentious and there are conflicting conclusions,” Goheen says. “So, we wanted to ask a similar question in a very different ecosystem. We thought, with this ecosystem (African savanna), we would be able to control for some of the issues that have proven difficult to address through the Yellowstone work.”
One of the reasons the Yellowstone studies have been so contentious is that other factors beyond wolf reintroduction also may be affecting dwindling elk numbers. They include drought, an increase in the number of grizzly bears and more hunting pressure on elk, Goheen says.
“Our study site didn’t have those confounding factors,” he says. “So, we thought this (Kenya) was an opportunity to see whether big, fierce carnivores could actually influence where different species of trees occurred.”